1. Field of the Invention
The present invention generally relates to a method for determining a position of a mobile communication terminal, and more particularly relates to a method for determining an initial position in a personal navigation system using global positioning system (GPS) satellites to reduce a radio positioning error and to improve positioning accuracy in radio positioning for determining a position of a mobile communication terminal.
2. Description of the Related Art
Conventionally, a navigation system is mounted in a vehicle to determine its position and provide an optimum route to a desired destination. This navigation system conventionally determines a position of a vehicle using a global positioning system (GPS).
Currently, the GPS satellite navigational system includes 24 satellites orbiting the Earth at an altitude of about 20.183 Km. GPS satellite navigation systems determine a position of a GPS receiver at a desired position by computing a time taken to receive radio waves (hereinafter called a GPS signals) transmitted by the GPS which are received by a GPS receiver.
Accordingly, conventional automotive GPS navigation systems include a GPS receiver and a GPS sensor mounted at a predetermined position on a vehicle equipped with the GPS navigation system. These navigation systems determine a vehicle's position by analyzing GPS signals received from four satellites. However, there are various positioning error factors to take into consideration when determining a vehicle's position. These error factors include an ionospheric delay error, a satellite clock error, multipath error, and so on. Specifically, GPS navigation systems compute an arrival time of a radio wave from a satellite to the receiver, and computes a position therefrom. When a GPS signal is interrupted or reflected by an object (e.g., tall building, a roadside tree, etc.) which is typical in a metropolitan area, the GPS signal usually cannot be accurately received, and GPS positioning errors increases.
A positioning error included in the GPS signal is about 5 m. Position error estimation on a two-dimensional plane is performed through a multiplication operation on a GPS signal and a horizontal dilution of precision (HDOP) value computed using a satellite constellation. Conventionally, the HDOP value is 2 to 3 when a high accuracy feature is absent. In this case, a position error in the range of 10 to 15 m can be estimated. The HDOP value may increase to 4 or more when a GPS receiver cannot receive a direct GPS signal transmitted from a satellite (due to an object blocking the GPS signal, etc.) and instead receives GPS signals from other more distant satellites. In this case, the position error increases to a value in the range of 20 to 30 m.
Because vehicles typically travel on mapped roads GPS vehicle navigation systems can match an initial position to a position on a given road by referring to a map database (DB) even though a position error is about 30 m. Because distances between roads can typically exceed 30 m, a GPS position error does not affect an initial position determination. However, in the case of a pedestrian walking on a sidewalk for example, two sidewalks which are adjacent to and located on both sides of a road a personal navigation system (PNS) therefore cannot accurately determine which sidewalk of two sidewalks the person is walking on if the distance between the road centerline and the sidewalk is about 20 m and if a position error is about 30 m.
The above-described example will be described with reference to FIG. 1 which illustrates an example in which an error in the conventional initial position determination occurs due to a GPS positioning error.
The pedestrian is present on the sidewalk 10. An example in which an object 20 (e.g., a tall building, a roadside tree, etc.) is located adjacent to the sidewalk 10 will be described. In this case, the distance between a road centerline 40 and a sidewalk 10 or a sidewalk 60 is conventionally about 20 m.
In FIG. 1, if the object 20 (e.g., a tall building, etc.) is located adjacent to the pedestrian when an initial position of the pedestrian is measured, a GPS signal from a satellite may be reflected or diffracted by the feature 20. Accordingly, because the signal is not a direct signal, a position error of a GPS receiver increases. When the position error has a radius of about 30 m or more from the pedestrian, it can be determined that the pedestrian is located at a position as indicated by reference numeral 50. Because the position 50 is present between the road centerline 40 and the sidewalk 60, the position of the pedestrian is actually measured near the sidewalk 60 opposite to the sidewalk 10. The PNS determines that the pedestrian is located on the opposite sidewalk 60, and determines an initial position based on the sidewalks 60 location.
Thus, using a conventional PNS system, when the pedestrian walks on a sidewalk adjacent to a tall building etc.), a position error may occur due to object interference, and a PNS the may erroneously determine that the pedestrian is located on an opposite sidewalk rather than on a target sidewalk when an initial position is determined which can inconvenience the user. When the initial position of the pedestrian is matched to the opposite sidewalk rather than the target sidewalk, a large error occurs in a route guidance function and the quality of route guidance service is degraded.